The present disclosure particularly relates to photomasks used for proximity exposure, and pattern formation methods and exposure apparatuses using the photomasks.
In microfabrication of semiconductor integrated circuits, MEMS processing, and surface processing of flat panels such as liquid crystal panels, lithography is used, in which pattern exposure is performed on a photosensitive resin layer formed on a substrate to be processed, and the exposed photosensitive resin layer is developed to form a desired pattern. In lithography, an object is generally irradiated with light via a photomask with a pattern. At this time, in order to reduce damages caused by close contact between a photomask and a body to be exposed, exposure is performed with the photomask spaced apart from the body to be exposed, i.e., proximity exposure is often performed.
However, in this case with the space, even if a pattern in a size about several times the wavelength is formed on a mask to form a fine pattern in a size about several times the exposure wavelength, a pattern in a desired size cannot be formed on a substrate due to the Fresnel diffraction etc.
For example, as shown in FIG. 23, in exposure using an exposure photomask 203 including a light-shielding film 202 formed on a transparent substrate 201, exposure light 204 transmitted by the photomask 203 is diffracted, thereby blurring a pattern. Even in projection exposure, in which a lens is provided between the photomask 203 and a substrate 205 to be processed, an opening 203a in a size about several times the wavelength of the photomask 203 does not transmit sufficient light in same-size projection exposure. Therefore, a fine pattern is difficult to form.
In recent years, reduced projection exposure has been suggested, in which a reducing lens is provided as the projection lens between the photomask 203 and the substrate 205, and a mask in a size four or five times the size after processing is used to increase the size of a pattern on the mask, thereby forming a fine pattern. However, in the reduced projection exposure, the entire exposure area is reduced to make processing of a large-area substrate difficult. In addition, the reduced projection exposure requires a plurality of high-precision lens for reduction, thereby increasing the costs for an apparatus for exposure.
In same-size projection exposure using proximity exposure or projection exposure, a method of forming a pattern in a desired size by increasing the intensity of light transmitted by a minute opening in a size about several times the wavelength.
Formation of a pattern, which is smaller than a limit for exposure by lithography using a photomask 203, will be described below with reference to FIGS. 24A and 24B. (See, for example, Japanese Patent Publication No. S57-054939)
First, as shown in FIG. 24A, the photomask 203 includes a glass substrate 207, and a light-shielding film 208 formed on the glass substrate 207 and having an opening 208a, which is a fine pattern portion. The photomask 203 includes a high refractive region 209, which has a higher refractive index than the glass substrate 207, in and near the region of the glass substrate 207, which is exposed from the opening 208a. Where the glass substrate 207 is made of, for example, silicon oxide, the refractive index is 1.5. When, for example, titanium (Ti) is added to the high refractive region 209, the refractive index becomes 1.8. The light-shielding film 208 is a vapor deposited film made of, for example, chrome (Cr).
As clear from FIG. 24B, the high refractive region 209 serves as a convex lens in the opening 208a, which is a light path of exposure light when being transmitted by the photomask 203. This collects the light passing through the region around the opening 208a so that the intensity of light transmitted by the fine pattern portion 208a becomes higher than the intensity of light passing through the normal mask portion. As a result, a fine pattern is formed.
Japanese Patent Publication No. S57-054939 shows that the high refractive region 209 is formed by ion implantation etc. to implant highly polarizable ions into and around the opening 208a of the light-shielding film 208 on the glass substrate 207.